Electrodynamic bifilar oscillograph for noiseless sound recording on films



Feb. 12, 1952 H, THIEMANN ELECTRODYNAMIC BIFILAR OSCILLOGRAPH FOR NOISELESS SOUND RECORDING ON FILMS Filed Feb. 14, 1948 Patented Feb. 12, 1952 ELECTRODYNAMIC BIFILAR OSCIL LO- GRAPH FOR NOISELESS :SOUND RE- CORDING 0N FILMS Hugo Thiemanmzurich, Switzerland, assignor to Gesellschaft zur 'Forderung der Forschung an der E. H., Zurich, Zurich, Switzerland, 2.

:firm :of Switzerland -Applicaliion February 14, 1948, Serial No. 8,446

InSwitzerland February 18, 1947 8 Claims. (01. 17.91-0.0.'3)

For sound-recording on motion picture films noiseless recording (pure tone or clear tone method) is generally adopted today, Whirihyasis well-known consists in that all areas of'the sound-track not required for recording sound appear dark in the positive. Thereby, during sound-reproduction, the disturbing extraneous noises mainly produced by damage of the said area-s and unavoidable after long use, are minimized. Of the methods hithertomade'known the light-beam trace of the sound-recording optics is, for instance, influenced at twodifieren't points, i. e. on the one hand by a sound-recording control device actuated by the sound-amplifier, and on the other hand "by a light control device influenced from the so-called noiseless' amplifier, which device causes the darkening of the-soundtrack areas not afiected by "sound-recording.

The devices and parts intended for actual soundrecording are referred to hereinafter simply by sound-system, and the parts adapted to exercise the noiseless functions are termed "noiseless system.

Another existing sound-recording method consists in that both the sound-amplifier as well as the noiseless amplifier are causedto act upon-one single light control device. "If a bifila'r "oscillograph is chosen as light control device, it follows thatthe oscillogra'ph loop is fed simultaneously from' the sound-amplifier and from the noiseless amplifier, but this may easily lead to thermal overloading of the loop.

The present invention relates to an electrodynamic bifilar :osci'llograph having a mirror 'for sound recordings with a shiftedzzero lirre, whereby the center of oscillations is steered :according to the sound oscillations to be recorded corre spending to the :desired shifting :of'the zero line. The invention is characterized by -the =provision of la rotatablezbo'dy carrying the celectrodynamic bifilar oscillograph system and turned by electrodynamic forces to produce the desired shifting metal strings 5a and 5b .of the bifilar loop 5, arranged closely parallelto each othenthat the string pair 5a and 5b located within the frame constitutes the signal recording system with the intermediately positioned very "light cemented-on mirror 1 characterized by its high resonance frequency (10,000 cycles), and that the string sections -5c and 5d located outside "the trance with the comparatively heavy frame 2 represent the mechanical vibratory or noislesssys'tem of very low torsional natural frequency (about v35 cycles) The string pair 511, 51; has the rectangular noiseless frame '2 clamped thereon bymeans of screws Ha and slots 12a so that the plane through the strings, the plane perpendicular to the direction of the strings and in the mirror centre between the rigidly clamped string ends, and @the plane lying between the strings in the direction thereof and perpendicular to the plane through the strings, represent symmetrical :planes with .relation to the -noiselessframe. The. strings of the loop 5, which are held firmly at ['6 and 11, pass through slots 12 in the noiseless frame 2- and should be clamped only 'to such an extent that upon =variations in the string mechanical string tension, the tension inside and outside the frame remains unchanged which is mounted in an immovable guide member 6. A bolt I3 is located in theg-uide member 8 and is prevented from turning 'bya pin M which holds'the loop5. Thus the spring I0 holds the string loop 5 under desired tension by means of the shiftable bolt 13. The bifilariloop is tensioned'by the spring Ill.

The magnetic combination of sound and noiseless system ensues in such away that the magnetic flux of apermanent'or'electro-magnet (not shown) is led through soft iron or cobalt iron poles 9 so that the magnetic lines of :force pass at right angles through the "noiseless .frame strings arranged parallel to the loop direction and the sound-recording system iinside theframe. In order to save magnetic tension, two iron pole pieces 3 are mounted :between the frame 2 and the string portions 5a, 5b therein '50 as to obtain three serially-connected air gaps l5a, 15b, 15c. Thenairgap 5|:5b of the bifilar 100115, with very high air gap induction, lies in the'centre and'the noiseless alrgap 15a, IE0 "at both sides "thereof, between which the noiseless frame 2 is allowed to move freely. The drive -.of the noiseless system takes place on the electrodynamic principle via a'winding :4 0f the noiseless frame, whilst the recording-system is fed via the bifilar loop'5. To prevent any lshort-circu'iting of the loop current through the noiseless frame, the current-inlet of 3 the loop at location 8 is insulated at the current supply end.

As a result of the noiseless recording, the ratio of useful to noise level when playing off films may be kept constant. As is known, the noise level caused by film and photo-cell grows with the amount of light striking the operating photocell. Attempts are therefore made to keep the average amount of control light exactly to that required for the momentary signal. This can be achieved if the electromechanical noiseless system fed from a separate electric control connection, termed noiseless amplifier, controls the average exposure of the film in such a manner that the negative amplitudes of the signal correspond with no exposure. Consequently in the noiseless amplifier a portion of the signal tension is fundamentally led on to the electromechanical noiseless system in the sound-optics, rectified, screened and amplified in proper measure.

According to the present invention the action of the bifilar oscillograph is as follows:

By reason of the noiseless frame the soundsystem located inside the frame is more or less totally turned with the movement of the frame, so that the middle position of the mirror 3 executing torsional vibrations on the inner string system 5a, 5b in rhythm with the sound-alterhating tension, follows the noiseless movements of the frame 2. With this arrangement one accomplishes that, for instance, when recording double jagged writing, an illuminated trianglediaphragm II as in Fig. 2 is in the sound-optics, through prism l5, oscillograph mirror l2 and through a lens IS in front of the latter, so repro- I duced in a slit diaphragm l4, that the length of the slit illuminated by the triangular form undergoes changes by the movement of the latter caused by the torsion of the mirror, in rhythm with the occurrence of sound, whereby the average length of the illuminated slit follows the movements of the noiseless frame, thus constantly assimilating, with appropriate noiseless adjustment, the average quantity of light to the average level of the occurrence of sound. n the film the slit illuminated by the triangular diaphragm is reproduced by an objective so that, due to constant travel of the film, the double jagged writing results on the variable area method.

By reason of the aperiodic transient effect of the noiseless system determined by the torsional elasticity of the string sections c, 5d outside the frame, the inertia of the frame 2, its eddy current-dampingin the air gaps a, I and the damping of a liquid possibly surrounding the whole oscillograph, the 15 ms. minimum transient time of the noiseless control, as established by experience, is ensured. The much longer chosen dying-away time, 300 ms., is electrically determined by an RC-member.

On account of their mechanical combination the noiseless system and the sound system represent a coupled mechanical vibratory assembly, in which two coupling frequencies with an intermediate zero position must fundamentally occur. However, since the ratio of the natural resonances of both single systems as well as the ratio of the vibrating masses thereof is very great, the two coupling frequencies practically coincide with the natural frequencies of the single systems. Nevertheless, at very low frequencies, the sound-sys tern by its reaction on the noiseless frame brings the latter to resonance which of course involves a change in the frequency-response curve of the sound-system. To obviate this possibly detrimental effect, a portion of the signal alternating current is led to the noiseless frame via its winding 4 so that a torque proportional to the current acts contrariwise to the torque of the soundsystem. Thus the reaction of the sound-system on the noiseless system can be compensated for, so that the noiseless frame does not follow the signal vibrations, and thereby the straight frequency-response curve is ensured to the lowest frequencies.

By lowering the noiseless natural frequency below 20 cycles the compensation could be dispensed with, since then the disturbance in the frequency-response curve of the sound-system would lie'below theaudible range of frequency. The extension of the transient time associated therewith will have to be again electrically shortened to the normal value, by adding-for instance, to the current supplied by the noiseless amplifier, which corresponds to the envelope curve of the sound alternating currenta further current proportional to the first differential quotient of the former one and, if expense is no consideration, a further additional current proportional to the second differential quotient.

The noiseless recording according to the Wellknown zero line shifting process as given by this combined sound-noiseless system requires a limitation of the clear sound modulation down to the value corresponding to the maximum signal level.- Since overmodulation due to sudden rise of the signal level can never be completely avoided, the noiseless amplitude, without limitation, would grow to such an extent that the signal recording would for the greater part occur outside the recording range. This limitation of the noiseless amplitude may take place in the noiseless ampli her, since, for instance, a diode biassed to a definite value is liable to limit the grid tension of a clear sound amplifier tube.

Finally the wiring diagram of the noiseless amplifier. as in Fig. 3 remains to be disclosed.

The signal tension for the control of the bifilar oscillograph is led in parallel wiring to the terminals 3| of the noiseless amplifier. The condenser I9 is loaded through transformer l1 and a full-waVe-rectifier tube l8,- and unloaded through potentiometer 20. The RC-member including condenser l9 and potentiometer 20 is so chosen that the time-constant amounts to about 300 ms. corresponding to the required noiseless closing time. On the other hand the condenser should be of such dimensions that upon sudden rise of the signal level the loading time is much shorter than the required opening time of 15 ms. of the noiseless system. Hence we obtain through the condenser IS a direct voltage corresponding to the momentary peak value of the sound alternating tension. The ratio of noiseless to signal current may be adjusted with the potentiometer 20. Provision of noiseless current limitation is made by means of the diode 22 and resistance 2|, since the diode 22 is biassed through potentiometer 23 thus limiting the grid tension of the amplifier tube 24 down to the potential value of the diode cathode.

The amplifier tube counter-coupled through the resistance 25 provides the feeding of the noiseless system connected to the terminals 28. The measuring instrument 2'! allows checking of the current each time passing the noiseless windin The zero point adjustment of the noiseless system is proviqfid by the aid of potentiometer 28 (the resistance 30a provides the current supply of the noiseless winding), whereas the potentiometer 29 brings about the adjustment of the above-mentioned additional signal current which compensates for the undesired action exerted by the sound-system upon the noiseless system. The direct current is supplied through the terminals 32. The resistances 33 and 34 supply the grid of the amplifier tube 24.

What I claim is:

1. An electrodynamic oscillograph for sound recordings with a shifted zero line, wherein the central position of the oscillations is actuated in accordance with the sound oscillations to be recorded corresponding to the desired shifting of the zero line, said oscillograph comprising, in combination, two strings forming a bifilar loop, a recording mirror carried by said strings, separate clamping means for said strings, a rotary metal frame symmetricall mounted on said strings on opposite sides of said mirror, and electrodynamic means rotating said frame and comprising a winding carried by said frame.

2. An oscillograph in accordance with claim 1, comprising means forming magnetic air gaps connected in a common magnetic circuit, said rotary frame being located in said air gaps, said air gaps being connected in series with the air gap of the bifilar loop.

3. An oscillograph in accordance with claim 1, wherein said electrodynamic means include means supplying to said Winding a counter-current proportional to the alternating current in the bifilar loop for compensating the torque exerted by said loop upon said frame at low frequencies.

4. An oscillograph in accordance with claim 1, wherein said electrodynamic means include means supplying to said winding an additional current proportional to the first differential quotient of the operating current determined by the envelope curve of the sound alternating current for the purpose of limiting the swinging period of the frame, which is determined by the torque and the elasticity of the mounting.

5. An oscillograph in accordance with claim 4 wherein said electrodynamic means further include means supplying to said winding a further additional current proportional to the second differential quotient.

6. A bifilar oscillograph for sound recordings with a shifted zero line, wherein the central position of the oscillations is actuated in accordance with the sound oscillations to be recorded corresponding to the desired shifting of the zero line, said oscillograph comprising, in combination, an oscillograph system comprising a bifilar loop, and a recording mirror; a rotary body connected with said system, and electrodynamic means rotating said rotary body, said rotary body being held solely by said bifilar loop, said rotary body comprising a metal frame clamped symmetrically on opposite sides of said mirror upon the strings of said bifilar loop, and a winding upon said frame; and separate means for tightening said strings.

7. A bifilar oscillograph for sound recordings witha shifted zero line, wherein the central position of the oscillations is actuated in accordance with the sound oscillations to be recorded corresponding to the desired shifting of the zero line, said oscillograph comprising, in combination, an oscillograph system comprising a bifilar loop, and a recording mirror; a rotary body connected with said system, and electrodynamic means rotating said rotary body, said rotary body being held solely by said bifilar lo0p, and a magnetic circuit which includes the magnetic air gap of the bifilar loop and another magnetic air gap in series with the first-mentioned air gap and containing said rotary body.

8. A bifilar oscillograph for sound recordings with a shifted zero line, wherein the central position of the oscillations is actuated in accordance with the sound oscillations to be recorded corresponding to the desired shifting of the zero line, said oscillograph comprising, in combination, an oscillograph system comprising a bifilar loop, and a recording mirror; a rotary body connected with said system, and electrodynamic means rotating said rotary body, said rotary body being held solely by said bifilar loop, and electrical means for limitin the rotation of said rotary body.

HUGO THIEMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,125,608 Gerlach Aug. 2, 1938 2,126,353 Gerlach Aug. 9, 1938 2,139,235 Kellogg Dec. 6, 1938 

